Extension API: Throwing a CancellationError does not dismiss the kernel picker.

[kevineger]

Originally reported this bug / brought it up in #16434 (comment). Figured opening a standalone issue is best.

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I've got a couple commands which ultimately don't resolve kernels (e.g. Open documentation and Upgrade account).

The API docs say Returning undefined or null will result in the previous UI being displayed for back-style behaviour ... and If however users exit the UI or workflow ... extensions are then expected to throw a CancellationError, else the previous UI will be once again.

In handling these non-resolving commands, I've tried both returning undefined, null and throwing a CancellationError, but it's not dismissing the picker. Instead the picker stays stuck in a pending/loading state.

Seems to be an issue only in handleCommand since throwing a CancellationError in provideCommands dismisses the kernel picker as expected.

Am I doing something wrong? Or misinterpreting the API?